I don't think that's correct. I think that renouncing citizenship is considered a "taxable event" equivalent to a sale, upon which the holdings are taxes as if you sold them, at the existing capital gains rates. So, if you have all cash, there's no penalty, but if you have stocks, the state plus federal rate could be pretty high, depending on the mix of short and long term holding times and the amount of embedded profit.
If anyone has more detailed info, please add!
It’s more complicated than that. For instance your pension benefits must be taxed, even if you are not yet getting them, and even if nobody knows what you will get as it depends on how long you live. So say you have a defined benefit pension that you want to tap when you are 62, and you abandon your citizenship whan you are 58, then you must immediately pay 30% of the actuarial value benefits in taxes. And this won’t count as taxes when you get your pension for the government of the jurisdiction you live in.
There is double taxation and injustice here.